Friday, April 09, 2010

Eating seaweed: Japanese gut microbiome bacteria are not like mine

My daughter left Korea when she was 13 months. She enjoys eating a seaweed snack, and Nori condiments. I wonder if she also carries these novel bacteria ... (Incidentally, the US coverage is misleading. This is not about sushi, it's about the seaweed called "nori".)

... The tools in question are genes that can break down some of the complex carbohydrate molecules in seaweed ... They are wielded by the hordes of bacteria lurking in the guts of every Japanese person ... Some gut bacteria have borrowed the seaweed-digesting genes from other microbes living in the coastal oceans....

... Within each of our bowels live around a hundred trillion microbes, whose cells outnumber our own by ten to one. This ‘gut microbiome’ act like an extra organ, helping us to digest molecules in our food that we couldn’t break down ourselves. These include the large carbohydrate molecules found in the plants we eat. But marine algae – seaweeds – contain special sulphur-rich carbohydrates that aren’t found on land. Breaking these down is a tough challenge for our partners-in-digestion. The genes and enzymes that they normally use aren’t up to the task.

Fortunately, bacteria aren’t just limited to the genes that they inherit from their ancestors. They can swap genes between individuals as easily as we humans trade money or gifts. This ‘horizontal gene transfer’ means that bacteria have an entire kingdom of genes, ripe for the borrowing. All they need to do is sidle up to the right donor. And in the world’s oceans, one such donor exists – a seagoing bacterium called Zobellia galactanivorans.

Zobellia is a seaweed-eater. It lives on, and digests, several species including those used to make nori. Nori is an extremely common ingredient in Japanese cuisine, used to garnish dishes and wrap sushi. And when hungry diners wolfed down morsels of these algae, some of them also swallowed marine bacteria. Suddenly, this exotic species was thrust among our own gut residents. As the unlikely partners mingled, they traded genes, including those that allow them to break down the carbohydrates of their marine meals. The gut bacteria suddenly gained the ability to exploit an extra source of energy and those that retained their genetic loans prospered...

.... [the human gut bacteria] B.plebeius seems to have a habit of scrounging genes from marine bacteria. Its genome is rife with genes that are more closely related to their counterparts in marine species like Zobellia than to those in other gut microbes. All of these borrowed genes do the same thing – they break down the complex carbohydrates of marine algae...

... To see whether this was a common event, Hehemann screened the gut bacteria of 13 Japanese volunteers for signs of porphyranases. These “gut metagenomes” yielded at least seven potential enzymes that fitted the bill, along with six others from another group with a similar role. On the other hand, Hehemann couldn’t find a single such gene among 18 North Americans....

... People might only gain the genes after eating lots and lots of sushi but Hehemann has some evidence that they could be passed down from parent to child. One of the people he studied was an unweaned baby girl, who had clearly never eaten a mouthful of sushi in her life. And yet, her gut bacteria had a porphyranase gene, just as her mother’s did.

... “Today, sushi is prepared with roasted nori and the chance of making contact with marine bacteria is low,” she said. The project’s other leader, Gurvan Michel, concurs. He notes that of all the gut bacteria from the Japanese volunteers, only B.plebeius as acquired the porphyranase enzymes. “This horizontal gene transfer remains a rare event,” he says....

... Rob Knight, a microbiome researcher from the University of Colorado... “This result reinforces the need to conduct a broad and culturally diverse survey of who harbours what microbes. The key to understanding obesity or IBD might well be in genes or microbes acquired under circumstances very different to those we experience in Western society.”

Because Nori is now cooked, and because persistence seems fragile, the genes will probably disappear from Japanese gut bacteria. It's a fascinating example, however, of the power of the microbiome. The therapeutic implications are obvious. Science fiction writers, incidentally, have long described the use of tailored gut bacteria to enable novel diets. When we run out of beef, cooked grass might be yummy.--My Google Reader Shared items (feed)